In both these works, the authors describe fictional events in a detailed, realistic style. The key difference is that Daniel Defoe, in Robinson Crusoe, wants to trick the reader into thinking the events in his novel really took place, whereas Jonathan Swift, in Gulliver's Travels, wants the reader to understand his story as an allegory.
The events of Robinson Crusoe are remarkable, fascinating, extraordinary--but totally possible in the real world. A man is stranded on a remote island and manages to survive through wit, hard work, and some good fortune. Defoe's genius is that he tells his story with such realistic detail that we are inclined to believe it as fact.
The events of Gulliver's Travels, by contrast, are obviously contrived: a land of midgets, a land of giants, a land of talking horses, etc. Although Swift does a great job of describing his inventions, no intelligent reader takes his stories as fact. Rather, they are parodies of various aspects of humanity.